1. Field of the Invention
The present invention relates to a radiographic-image recording medium which records radiographic-image information by storing electric charges generated in response to exposure to radiation.
2. Description of the Related Art
The following documents (1) and (2) disclose information related to the present invention.
(1) U.S. Pat. No. 6,268,614 issued to the present inventor (Shinji Imai) and corresponding to Japanese Patent Applications Nos. 10-215378 and 10-232824 (which are laid open as Japanese Unexamined Patent Publication No. 2000-105297)
(2) U.S. Pat. No. 6,121,620 corresponding to Japanese Patent Application No. 8-34903 (which is laid open as Japanese Unexamined Patent Publication No. 9-230054)
Conventionally, radiographic-image recording mediums, which record radiographic images by storing in a charge storage region electric charges the amounts of which respectively correspond to doses of radiation such as X-rays, are used in many applications such as medical radiography. Various types of radiographic-image recording mediums basically operating as above have been proposed.
In aforementioned document (1), the present inventor has proposed a radiographic-image recording medium which can concurrently realize high speed response in reading and efficient readout of signal charges. In the proposed radiographic-image recording medium, a first electrode layer, a recording-side photoconductive layer, a charge transportation layer, a reading-side photoconductive layer, and a second electrode layer, stacked in this order. The first electrode layer is transparent to radiation for use in recording or light emitted due to excitation by the radiation for recording, the recording-side photoconductive layer becomes conductive when the recording-side photoconductive layer is exposed to the above radiation or light, the charge transport layer behaves substantially as an insulator against latent-image charges and substantially as a conductor of charges having the opposite polarity to the latent-image polarity, the reading-side photoconductive layer exhibits conductivity when the reading photoconductive layer is exposed to an electromagnetic wave for reading, and the second photoconductive layer is transparent to the electromagnetic wave for reading. Thus, when the radiographic-image recording medium is irradiated through the first electrode layer with the radiation for recording, a radiographic image is recorded by storing in a charge storage region electric charges the amounts of which correspond to the doses of the radiation. The charge storage region is formed substantially at an interface between the recording-side photoconductive layer and the charge transport layer.
Further, in document (1), the present inventor has also proposed a radiographic-image recording medium in which a wavelength conversion layer containing a fluorescent material (CsI) is arranged, where the fluorescent material emits visible light in the blue wavelength range in response to exposure to radiation for recording.
In each of the above radiographic-image recording mediums disclosed in document (1), information on the radiographic image recorded in the radiographic-image recording medium can be read out by scanning the radiographic-image recording medium with a laser light beam or a light beam having a linearly-shaped cross section.
Meanwhile, aforementioned document (2) discloses a radiographic-image recording medium includes a fluorescent sheet and a glass substrate on which a plurality of photoelectric conversion elements are arranged, and the glass substrate and the fluorescent sheet are bonded together. Each photoelectric conversion element on the glass substrate is constituted by a photodiode and a TFT (thin-film transistor) switch, and the fluorescent sheet emits fluorescent light in response to exposure to radiation. In this radiographic-image recording medium, each photodiode photoelectrically converts the fluorescent light emitted from the fluorescent sheet into an electric charge. Then, the electric charge is stored and read out by controlling the TFT switches on and off.
The above radiographic-image recording mediums are mainly used in hospitals and the like. It is desirable that each radiographic-image recording medium is contained in a cassette or the like so as to be portable, and is shock resistant so that the radiographic-image recording medium is not damaged even when the radiographic-image recording medium is unintentionally dropped. Further, since the radiographic-image recording mediums may be used outside the hospitals, for example, in a mobile van service, there are intense demands for portability and shock resistance of the radiographic-image recording mediums.
However, the radiographic-image recording mediums disclosed in documents (1) and (2) have the following drawbacks (i) and (ii).
(i) The recording-side photoconductive layer in each radiographic-image recording medium disclosed in document (1) is made of a-Se. The thickness of the a-Se film required for satisfactorily detecting applied radiation is as much as about 1,000 micrometers. a-Se films having such a thickness are susceptible to damage when dropped. When the aforementioned wavelength conversion layer is used, the thickness of the recording-side photoconductive layer can be reduced, and the recording-side photoconductive layer can be made less susceptible to damage. However, since the above wavelength conversion layer is produced by vapor deposition of CsI, which forms a needle crystal, the wavelength conversion layer per se is very susceptible to damage.
(ii) In the radiographic-image recording medium disclosed in document (2), in order to form the TFTs on the substrate, the substrate is required to be made of heat resistant non-alkali glass or quartz glass, and the thickness of the substrate is required to be 0.7 to 3 mm. However, such a glass substrate also breaks easily upon impact, such as that applied when dropped.